The present invention generally relates to level sensing apparatus, and more particularly to fluid level sensors used in mixing devices.
Various types of commercial processes require a supply of liquid, which is commonly provided in a vessel. As the commercial process continues, the level of liquid in the vessel drops. It is important in many instances to maintain a desired level of liquid within the vessel. In a single fluid printing press which uses a pre-mixed ink/water emulsion to form images on paper, for example, a source of ink/water emulsion must continuously supply the fluid during the course of the printing operation. Such presses include a vessel which houses a mixing device for blending the ink and water. The mixing device typically rotates on the order of 1000-2000 rpm to vigorously and thoroughly mix the ink and water to obtain the ink/water emulsion. Accordingly, the vessel is substantially closed, to prevent fluid from leaking. The ink/water emulsion is then supplied to the press to print images on paper.
As is known in the art, it is common to provide a liquid level sensor to provide feedback regarding the amount of fluid in the vessel. Most commonly, control systems for maintaining the level of liquid in a vessel have utilized a float element that is supported on the surface of the liquid. This type of control system depends upon the float element riding up and down as the level of the liquid fluctuates within the vessel to activate respective off and on switches to control suitable valves for selectively replenishing the liquid from a remote source. More specifically, the actuation of the switches that control the valves is typically accomplished either mechanically or by energizing a proximity sensing device. Generally speaking, such control systems are well proven and quite reliable for most applications. They are known to be unsuitable, however, for use in an ink environment, due substantially to the fact that ink has a high viscosity and tack which causes it to adhere to the float element. Accordingly, the buoyancy and weight of the float element in this type of control system is known to change dramatically. As a result, the control system is known to be difficult, at best, to maintain in calibration within an ink environment.
An alternative float-type provides a regulated supply of low pressure air to the vessel through an air supply tube. A bottom end of the air supply tube is positioned at a preselected level within the liquid vessel. An air flow meter having a pressure-responsive floating element in an internal chamber is connected to the air supply tube to sense fluctuations in air pressure. The floating element is movable between a first position and a second position depending upon the air pressure which is sensed in the air supply tube. When the liquid level in the vessel is above the preselected level, the air supply tube is under a higher pressure and the float element of the air flow meter is in the first position. When the liquid level drops below the preselected level, the pressure in the air supply tube drops and the float element of the air flow meter moves to the second position. A liquid supply valve operates according to the position of the float element, so that it is closed when the float element is in the first position, and is open when the float element is in the second position. While this alternative removes the float element from the vessel to avoid the problems noted above, the ink tends to clog the air supply tube, rendering the flow meter useless. As a result, a supply of high pressure air is often needed to unclog the tube by blasting air through the tube. This approach not only requires additional components, but must also be used with open or vented vessels so that the pressurized air supplied to the vessel may escape, and therefore is not suitable for single-fluid printing press applications.
U.S. Pat. No. 5,694,974 describes yet another type of fluid level detection system. The system of the ""974 patent comprises a vessel for collecting ink. A rotating shaft is disposed inside the vessel, and upper and lower flexible blades are attached to the rotating shaft. Upper and lower proximity sensors are positioned at the respective heights of the upper and lower blades. When one of the blades is unimpeded by ink, the blade remains straight and actuates the associated proximity sensor as it passes the sensor. When the level of ink in the vessel reaches a blade, that blade flexes due to the viscosity of the ink, to form a greater gap between the proximity sensor and the blade, such that the sensor does not detect the blade. It will be appreciated that for such a system to be effective, the ink must be relatively inert or unmoving so that the blades will bend when submerged in ink. If the shaft rotates too fast, such that the ink circulates in with the shaft, the blades will not flex when submerged. In such systems, therefore, the shaft typically rotates on the order of 60 rpm, which is not suitable for mixing ink and water as needed in a single fluid printing press.
Ultrasound instruments have also been used to control the level of liquid in a vessel. The ultrasound instruments function by detecting an acoustic wave which is reflected from a surface of the liquid in the vessel and by measuring the time which elapses between the emission and detection of the wave thereby to calculate the distance of the liquid surface from the instrument. Unfortunately, the ultrasound instruments are quite unreliable in an ink environment inasmuch as the ink surface is quite irregular, especially when mixed vigorously, thereby preventing a proper reflection of the acoustic wave.
The present invention is directed to overcoming one or more of the foregoing problems and achieving one or more of the resulting objects.
According to certain aspects of the present invention, apparatus is provided for detecting a fluid level in a tank of circulating fluid. The apparatus comprises a paddle blade supported inside the tank at a particular level, the paddle blade oriented to resist fluid circulation in the tank and moveable between a normal position and a displaced position. The apparatus further comprises a detector for sensing when the paddle blade is in at least one of the normal and displaced positions and generating a fluid level signal. The circulating fluid acts to move the paddle blade from the normal position to the displaced position so that the fluid level signal is generated responsive to the level of the circulating fluid.
In accordance with additional aspects of the present invention, apparatus for mixing fluid is provided comprising a tank for holding fluid, a rotatable shaft supported inside the tank, and an impeller attached to the shaft for mixing fluid in the tank, thereby circulating the fluid. A paddle blade is supported inside the tank at a particular level, the paddle blade oriented to resist the circulating fluid in the tank and being moveable between a normal position and a displaced position. A detector is provided for sensing when the paddle blade is in at least one of the normal and displaced positions and for generating a fluid level signal. The paddle blade is adapted to move toward the displaced position when the circulating fluid reaches the particular level.
In accordance with additional aspects of the present invention, a method is provided of sensing the presence of circulating fluid in a tank at a particular height. The method comprises the steps of supporting a paddle blade inside the tank at the particular height, the paddle blade moveable between a normal position and a displaced position, and detecting when the paddle blade is in at least one of the normal and displaced positions and generating a signal. The paddle blade will move from the normal position to the displaced position when the circulating fluid is present at the particular height in the tank.